The invention is based on a fuel injection pump for internal combustion engines as generically defined hereinafter.
In slide-controlled fuel injection pumps of this type, the injection quantity and/or the injection onset is determined by the axial position of the control slide. There are also versions in which in addition to axial displacement, the control slide can also be rotated, to vary the fuel control in this way as well. In each case, however, the end of the high-pressure injection--which in terms of timing represents the end of injection, on the one hand, and on the other hand represents the end of high-pressure supply, determining the injection quantity--is determined by the uncovering of the control opening in the pump piston by the control slide, so that the fuel, which is at very high pressure, is diverted out of the pump work chamber via the relief conduit and the control opening into the recess of the cylinder liner. When diverted in this way, by means of a control edge of the control slide, the diverted fuel stream has extraordinarily high kinetic energy, which puts great strain on the materials on which the fuel stream impacts. Pressures in the pump work chamber can certainly be as high as 1300 bar. The energy losses between the pump work chamber and the diversion point are relatively slight, because upon diversion, the injection is abruptly interrupted, to assure fast nozzle needle closure.
In the control slide of a known fuel injection pump of this type (German Offenlegungsschrift No. 35 40 052) there is a radial diversion bore, which is uncovered, once the effective injection stroke has been executed, by an oblique control groove disposed as a control opening on the pump piston. At the time of diversion, the diverted stream shoots through this diversion opening onto the wall of the cylinder bushing recess. When the axial position of the control slide changes, the location of the diverted stream with respect to the point in the wall of the recess upon which the stream impacts changes as well. Since the wall is spaced apart from the mouth of the diversion bore only by the width of a gap, the stream is reflected from the wall into the gap and enters the suction chamber of the pump with very high energy.
This type of stream deflection has the disadvantage that the deflected stream, which still has very high kinetic energy, strikes the wall of the suction chamber--which like the entire pump housing is made of softer material, e.g., aluminum, than the pump cylinder liner and control slide, which are made of tempered steel. The result is cavitation and erosion damage to the suction chamber wall of the pump housing and to the control elements located in the suction chamber.
For in-line injection pumps, in which the diversion bore is always located at the same point, it is known (German Offenlegungsschrift No. 31 36 751) to provide an impact protection ring of tempered steel between the pump housing and the diversion bore. This kind of arrangement cannot be adopted for slide-controlled pumps, however, because the diversion bore shifts when the axial position of the control slide changes, and the gap between the control slide and the wall of the recess must always communicate, unthrottled, with the suction chamber, to prevent any undesirable effect on the fuel control during the intake stroke or even during the diversion. Also, an additional impact protection element would weaken the pump cylinder liner, or else the space required for the pump structure would have to be enlarged excessively.